Reactor chambers provide a controlled location for chemical processes using introduced vapor. Depending on the process, the vapor is ideally free of particles and droplets to ensure quality. Filters serve to reduce particles and droplets from entering the reactor chambers.
There are many particle filters available for chemical vapor deposition (“CVD”) reactors. CVD reactors are based upon a static flow of precursor vapor and the flow resistance of the filter is not especially important.
Atomic layer deposition (“ALD”), formerly known as atomic layer epitaxy (“ALE”), is a thin film deposition process based on dynamic flows. The ALD process relies on sequential pulsing of two or more precursor vapors over a substrate in a reaction chamber. To increase the productivity of an ALD reactor, it is advantageous to switch the precursor vapors as fast as possible. The films yielded by the ALD technique have exceptional characteristics, such as being pinhole free and possessing almost perfect step coverage.
A key to successful ALD growth is to have the correct precursor vapors pulsed into the reaction chamber sequentially and without overlap. Since the actual pulses are not Delta functions (i.e., do not exhibit instantaneous rise and decay), they will overlap if the second pulse is started before the first is completely decayed. Since both highly reactive precursor vapors are present in the reaction chamber at the same time, this condition leads to non-ALD growth, and typically CVD-type growth, which can lead to film thickness non-uniformity. To prevent this problem, the pulses must be separated in time.
High-resistance elements, such as particle filters, in the flow path from the main precursor switching element to the reaction chamber can result in much longer exponential decays in the precursor pulses. With a poorly designed precursor delivery system, it is common for the purge times, defined as the time between precursor pulses, to be 10 times as long as the pulse itself to prevent overlap of the precursor pulses and achieve good film thickness uniformity. Longer purge times increase processing time, which substantially reduces the overall efficiency of the ALD reactor. To optimize the throughput of a reactor and minimize particle generation, it is, therefore, desirable to create a precursor delivery system that has the fastest possible rise and decay of the precursor pulse.
For most films grown by ALD, particles in or on the film will reduce the manufacturing yield. It is, therefore, important that the precursor source does not emit any particles. This is especially difficult when the precursor exists in powder form at standard temperature and pressure (“STP”). Powdered precursors are changed to vapor by exposing the vapor to high temperatures and low pressure. The resulting vapor contains more contaminant particles than precursors that exist in liquid or solid phase at STP, because it is difficult to eliminate contaminant particles from a powdered mixture.
A typical solution is to add a high efficiency particle filter, which is quite common for CVD systems. These filters can typically block 99.99999% of particles smaller than 0.003 microns. However, such particle filters are very resistive to flow, which leads to long precursor decay times and, therefore, long process times.
U.S. Pat. No. 6,354,241 to Tanaka et al. and U.S. Pat. No. 5,709,753 to Olson et al. disclose filters that rely on sinuous flow paths to capture undesired materials. However, materials captured in the disclosed filters are continuously exposed to the flow and may be drawn back into the flow. Thus, these filters may not provide the required filtering efficacy.
High efficiency particle filters serve well in CVD systems, which rely on static vapor flow, but have limited use with ALD systems due to the dynamic nature of the ALD process. The present inventors have recognized that efficient filters having high flow conductivity are desirable for pulsed precursor vapor delivery systems.